The present invention relates to a method for controlling a four wheel drive vehicle. More particularly, the present invention relates to a method for controlling a distribution ratio of driving torque between front wheels and rear wheels, which includes a plurality of continuously variable transmissions.
A conventional four wheel drive includes a wet type clutch disc mechanism located between an output shaft of a transmission and a front differential gear mechanism. When the hydraulic pressure supplied to the wet type clutch disc mechanism is varied, the maximum transmittable torque of the wet type clutch disc mechanism is likewise varied. As a result, the distributed torque to the front wheels is adjustable. A conventional four wheel drive of this type is disclosed, e.g., in a publication "Motor Fan", dated Dec., 1985.
However, the distribution ratio of such conventional four wheel drives can only be changed when the wet type clutch disc mechanism is disengaged. When the wet type clutch disc mechanism is engaged, the conventional four wheel drive cannot change the distribution ratio of driving torque.
Accordingly, when the hydraulic pressure supplied to the clutch disc mechanism is great and a plurality of clutch discs cannot rotate within the clutch disc mechanism, the distribution ratio of the driving torque between the front wheels and the rear wheels becomes one to one.
When a different distribution ratio of driving torque is required, the plurality of clutch discs are allowed to rotate within the clutch disc mechanism. However, such rotation results in problems, such as abrasion or thermal damage of the clutch disc. Further, rotation of the clutch discs results in energy loss, thereby reducing the drive force transmittal line efficiency.
Furthermore, when the hydraulic pressure supplied to the clutch disc mechanism is constant (i.e., the maximum transmittable torque is constant) and the engine torque is increased, the transmitted torque to the front wheels is constant and the transmitted torque to the rear wheels increases. As a result, the distribution ratio of the driving torque varies.
However, in this condition, the maximum transmittable torque should be controlled to maintain a constant value of the distribution ratio of the driving torque. Therefore, the hydraulic pressure supplied to the clutch disc mechanism must be determined by not only the desired distribution ratio of the driving torque but also by the amount of the engine torque. Accordingly, it is complicated to control the hydraulic pressure supplied to the clutch disc mechanism, because the amount of the engine torque changes continuously. Therefore, it is difficult to control the distribution ratio of the driving torque between the front wheels and the rear wheels of the conventional four wheel drive with the wet type clutch disc mechanism.
Japanese Patent Laid Open Sho No. 59-151661 discloses an apparatus for changing the distribution ratio of the driving torque. The disclosed apparatus distributes driving torque of an engine between the front wheels and rear wheels, respectively, through a differential gear mechanism, and includes two continuously variable transmissions transmitting the driving torque, respectively, to both of the front and rear wheels. Accordingly, the apparatus can vary the speed ratio between the front wheels and the rear wheels, and adjust the distribution ratio of the driving torque. The differential gear mechanism of the apparatus absorbs any difference in rotational speed caused by the difference of the speed ratio.
Further, the apparatus includes sensors detecting a driving condition of the vehicle, and controls the distribution ratio of the driving torque in accordance with the detected driving condition. However, the specific method for controlling the distribution ratio of the driving torque in accordance with the detected driving condition is not clearly disclosed.